PSI - Issue 23

Pavol Dlhý et al. / Procedia Structural Integrity 23 (2019) 185–190 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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(2013)). Fatigue cracks are most likely to initiate at the axle surface (because of presence of maximal bending stress, notch effect, environmental influence, etc., see e.g. Pokorný et al. (2014) or Luke et al. (2010)). Such cracks can propagate and cause fatigue failure of a railway axle in service with serious consequences. To prevent such situation, engineers try to develop axle design and technology to improve fatigue properties. One of the technology procedures for improvement of fatigue properties is surface heat treatment which induces residual stress in axle. This paper considers induction hardened axle which has compressive residual stresses at axle surface area. Compressive stress at the surface is important because the most of residual fatigue lifetime of the fatigue crack growth is usually consumed by growth up to 5% of critical crack length (Náhlík et al. (2017)). Influence of residual stress at residual fatigue lifetime was shown by Hutař et al., (2017) . The problem is that residual stresses cannot be measured directly in the whole cross section. Commonly used non-destructive methods for determination of residual stress are limited by the depth in which stresses can be measured (Schajer (2013)). Therefore, to determine residual stress through the whole cross-section of railway axle, it is necessary to use some of the destructive methods (e.g. removing layer by layer technique with final recalculation). It is necessary to point out that each cut of axle causes residual stresses to redistribute and the original stresses must be recalculated then. In literature, several ways how to determine tangential (also called hoop or circumferential) residual stress can be found (Guillen et al. (1998); Akbari et al. (2013); Taran et al. (2008); Schajer (2013)). Slit ring method was chosen because of its simplicity and available equipment. This method was compared with neutron and x-ray diffraction techniques. Method of the slit rings is a method to determine tangential residual stress through the whole cross-section in hollow cylinder parts. The procedure is well described in papers Poduška et al. (2014) and Poduška et al. (2016). The scheme of the procedure is pictured in Fig. 1. To determine the tangential residual stress in a railway axle using the method of slit rings, it is necessary to cut out part of the axle and drill a hole in it. It is assumed that the wall of this hollow cylinder consists of a certain number of layers in which the tangential residual stress is constant. Then, this part is cut into rings. The total number of rings is related to the number of layers assumed and it affects the precision of the latter stress evaluation. Each ring is then specifically modified by removing the layers of material using a lathe. Then, a segment is cut out of all the rings to allow their deflection. The deflection (change in diameter) is measured. 2. Modification of the methodology

Fig. 1 Schematic illustration of procedure for determination of tangential residual stress in railway axle by slit ring method based on curved beam theory.